U.S. patent application number 14/236480 was filed with the patent office on 2014-07-17 for solder compositions.
This patent application is currently assigned to ALPHA METALS, INC.. The applicant listed for this patent is Ravindra M. Bhatkal, Kamanio Chattopadhyay, Morgana de Avila Ribas, Dominic Lodge, Proloy Nandi, Ranjit Pandher, Rahul Raut, Siuli Sarkar, Bawa Singh. Invention is credited to Ravindra M. Bhatkal, Kamanio Chattopadhyay, Morgana de Avila Ribas, Dominic Lodge, Proloy Nandi, Ranjit Pandher, Rahul Raut, Siuli Sarkar, Bawa Singh.
Application Number | 20140199115 14/236480 |
Document ID | / |
Family ID | 46826859 |
Filed Date | 2014-07-17 |
United States Patent
Application |
20140199115 |
Kind Code |
A1 |
de Avila Ribas; Morgana ; et
al. |
July 17, 2014 |
SOLDER COMPOSITIONS
Abstract
A solder composition comprising a blend of a first powder
component and a second powder component, wherein the first powder
component is a first solder alloy and the second powder component
is a second solder alloy or a metal.
Inventors: |
de Avila Ribas; Morgana;
(Bangalore, IN) ; Lodge; Dominic; (Woking, GB)
; Pandher; Ranjit; (Plainsboro, NJ) ; Singh;
Bawa; (Voorhees, NJ) ; Bhatkal; Ravindra M.;
(East Brunswick, NJ) ; Raut; Rahul; (Edison,
NJ) ; Sarkar; Siuli; (Bangalore, IN) ;
Chattopadhyay; Kamanio; (Bangalore, IN) ; Nandi;
Proloy; (Bangalore, IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
de Avila Ribas; Morgana
Lodge; Dominic
Pandher; Ranjit
Singh; Bawa
Bhatkal; Ravindra M.
Raut; Rahul
Sarkar; Siuli
Chattopadhyay; Kamanio
Nandi; Proloy |
Bangalore
Woking
Plainsboro
Voorhees
East Brunswick
Edison
Bangalore
Bangalore
Bangalore |
NJ
NJ
NJ
NJ |
IN
GB
US
US
US
US
IN
IN
IN |
|
|
Assignee: |
ALPHA METALS, INC.
South Plainfield
NJ
|
Family ID: |
46826859 |
Appl. No.: |
14/236480 |
Filed: |
August 2, 2012 |
PCT Filed: |
August 2, 2012 |
PCT NO: |
PCT/GB2012/051876 |
371 Date: |
March 21, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61514396 |
Aug 2, 2011 |
|
|
|
Current U.S.
Class: |
403/272 ; 148/24;
228/101; 228/256; 228/56.3; 75/252; 75/255 |
Current CPC
Class: |
Y10T 403/479 20150115;
B23K 35/025 20130101; B23K 35/3026 20130101; B23K 35/262 20130101;
B23K 35/3601 20130101; B23K 35/3033 20130101; B23K 35/0244
20130101; B22F 1/0003 20130101; B23K 35/24 20130101; B23K 35/3013
20130101; B23K 35/0233 20130101; B22F 1/0074 20130101; B23K 35/264
20130101; B23K 35/3006 20130101; B23K 35/302 20130101 |
Class at
Publication: |
403/272 ; 75/255;
75/252; 148/24; 228/56.3; 228/101; 228/256 |
International
Class: |
B23K 35/24 20060101
B23K035/24; B23K 35/02 20060101 B23K035/02 |
Claims
1. A solder composition comprising a blend of a first powder
component and a second powder component, wherein the first powder
component is a first solder alloy and the second powder component
is a second solder alloy or a metal.
2. The solder composition according to claim 1, wherein the solder
composition consists of the blend of the first powder component and
the second powder component, together with unavoidable
impurities.
3. The solder composition according to claim 1, wherein the solder
composition is lead-free.
4. The solder composition according to claim 1, wherein the blend
comprises the first and second solder alloys and wherein the first
and second solder alloys comprise at least one common element.
5. The solder composition according to claim 4, wherein the at
least one common element is tin.
6. The solder composition according to claim 1, wherein the blend
comprises the first and second solder alloys and wherein the first
and second solder alloys have different melting points.
7. The solder composition according to claim 6, wherein the melting
points differ by at least 5.degree. C.
8. The solder composition according to claim 6, wherein the first
powder component forms about 80% by weight of the solder
composition and is 42Sn 58Bi, and wherein the second powder
composition forms about 20% by weight of the solder composition and
is SAC305 (96.5% Sn, 0.5% Cu, 3% Ag).
9. The solder composition according to claim 1, wherein the second
alloy or metal comprises an element selected from Cu, Ag, Al, Au,
Cr, In, Sb, Sc, Y, Zn, Ce, Co, Ge, Mn, Ni and Ti or rare earth
elements.
10. The solder composition according to claim 9, wherein the powder
components comprise metal particles which are: (i) from 1 nm to 100
microns; or (ii) from 10 nm to 100 microns; or (iii) from 100
microns to 1000 microns.
11. The solder composition according to claim 1 comprising the
first and second solder alloys, wherein the first and second solder
alloys have similar melting temperatures and are immiscible.
12. The solder composition according to claim 11, wherein the
melting temperatures of the first and second solder alloys are
within 10.degree. C.
13. The solder composition of claim 11, wherein the coefficient of
thermal expansion of the first solder alloy is positive and wherein
the coefficient of thermal expansion of the second solder alloy is
negative.
14. The solder composition of claim 11, wherein the second powder
component has a non-reactive coating layer.
15. The solder composition according to claim 1, further comprising
a further powder component selected from a carbide, a nitride, an
oxide or carbon nanotubes, preferably selected from
Al.sub.2O.sub.3, SiO.sub.2, TiO, NiO and carbon nanotubes.
16. A solder composition comprising a blend of a first powder
component and a second powder component, wherein the first powder
component is a first solder alloy and the second powder component
is selected from a carbide, a nitride, an oxide or carbon
nanotubes, preferably selected from Al.sub.2O.sub.3, SiO.sub.2,
TiO, NiO and carbon nanotubes.
17. The solder composition of claim 1 in the form of a solderable
paste, a film, a strip, a foil, a wire, a preform or a sphere.
18. (canceled)
19. (canceled)
20. Forming a joint using the composition of claim 1 in a soldering
method.
21. (canceled)
22. A soldered joint formed from the blend of claim 1.
23. A solder composition comprising a blend of a first component
and a second component, wherein the first component is a first
solder alloy and the second component is a second solder alloy or a
metal.
24. The solder composition of claim 23 wherein the first component
is in the form of a paste and the second powder component is in the
form of a powder, paste, strips, foils, spheres, discs or a
preform.
25. A method according to claim 20 comprising: (i) providing two or
more work pieces to be joined; (ii) providing a first solder
component having a first reflow temperature; (iii) providing a
second solder component having a second reflow temperature that is
higher than said first reflow temperature; and (iv) heating said
first and second solder components in the vicinity of the work
pieces to be joined, wherein said heating is carried out at or
above the first reflow temperature and below the second reflow
temperature.
Description
[0001] The present invention relates to a solder composition, in
particular to a lead-free solder composition. The solder
composition is comprised of two or more components to provide
improved characteristics to the solder.
[0002] Lead-free solder alloys are well known and provide non-toxic
alternatives to the most widely used solder alloy-eutectic 37%
Pb-63% Sn alloy. Examples of such lead-free alloys include the
binary eutectic 58% Bi-42% Sn alloy (see, for example, U.S. Pat.
No. 5,569,433 B) and the binary 40% Bi-60% Sn alloy (see, for
example, U.S. Pat. No. 6,574,411 A). Such alloys exhibit a loss of
ductility at high strain rates, which can be improved by the
addition of small amounts of additives, such as up to 1% by weight
silver (see, for example, U.S. Pat. No. 5,569,433 B). However, the
impact energies exhibited by these alloys, measured using the
Charpy Impact Test, are relatively low. Accordingly, there is a
need to develop lead-free solder alloys which exhibit improved
impact toughness.
[0003] In order for such lead-free alloys to be used in soldering
methods such as wave and reflow soldering, the alloys must exhibit
good wettability in relation to a variety of substrate materials
such as copper, nickel and nickel phosphorus ("electroless
nickel"). Such substrates may be coated to improve wetting, for
example by using tin alloys, silver, gold or organic coatings
(OSP). Good wetting also enhances the ability of the molten solder
to flow into a capillary gap, and to climb up the walls of a
through-plated hole in a printed wiring board, to thereby achieve
good hole filling.
[0004] Furthermore, solder compositions need to exhibit a good
thermal fatigue life and decreased high temperature creep. Improved
ductility and thermal and electrical conductivity are also
desirable. These properties can be achieved by selecting a specific
solder alloy if one is known, or through the use of specific
additives. However, it would be advantageous if the properties of
existing commonplace solders could be adapted to provide these
benefits without requiring alternative solder alloys to be
developed.
[0005] Accordingly, there is a desire for a solder composition that
will overcome, or at least mitigate, some or all of the problems
associated with the solders of the prior art or at least a useful
or optimized alternative.
[0006] According to a first aspect, the present invention provides
a solder composition comprising a blend of a first powder component
and a second powder component, wherein the first powder component
is a first solder alloy and the second powder component is a second
solder alloy or a metal.
[0007] The present disclosure will now be further described. In the
following passages different aspects of the disclosure are defined
in more detail. Each aspect so defined may be combined with any
other aspect or aspects unless clearly indicated to the contrary.
In particular, any feature indicated as being preferred or
advantageous may be combined with any other feature or features
indicated as being preferred or advantageous.
[0008] The term "solder alloy" used herein refers to a fusible
metal alloy with a melting point in the range of from 90-400
degrees C.
[0009] The "Charpy impact test" referred to herein, also known as
the Charpy v-notch test, is a standardized high strain-rate test
which determines the amount of energy absorbed by a material during
fracture. This absorbed energy is a measure of a given material's
toughness and acts as a tool to study temperature-dependent
brittle-ductile transition. Further details regarding this test can
be found in Charpy Impact Test: Factors and Variables, J. M. Holt,
ASTM STP 1072, the contents of which is hereby incorporated by
reference.
[0010] The term "wettability" used herein refers to the degree to
which solder spread on a wettable surface. Wettability is
determined by surface tension of the liquid solder and its ability
to react with the wettable surface. Wetting can also be described
in terms of the contact angle of the molten and subsequently frozen
solder alloy on a substrate, with lower contact angles being
favoured over high contact angles.
[0011] The term "wave soldering" used herein refers to the
large-scale soldering process by which electronic components are
soldered to a printed circuit board (PCB) to form an electrical
assembly.
[0012] The term "reflow soldering" used herein refers to the
process where solder paste is printed or dispensed, or a solder
perform is placed on the surface of a printed circuit board,
components are placed in or near the deposited solder, and the
assembly is heated to a temperature above the liquidus of the
solder alloy.
[0013] The term "rare earth element" used herein refers to an
element selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb,
Dy, Ho, Er, Tm, Yb and Lu.
[0014] All percentages herein are by weight unless expressly stated
otherwise.
[0015] Preferably at least one of the powders is spherical,
preferably both. That is, at least 90% of the particles have a
length-to-width ratio of less that 1.5. Preferably at least 95%,
more preferably at least 98%, of the particles have a
length-to-width ratio of less that 1.5. For most applications this
high degree of "sphericity" is preferred, the major advantage being
lower surface area, which minimizes oxidation, and better load
acceptance (less tendency for clogging and interlocking), which
assists dispensability and release through a stencil aperture. In
an alternative embodiment, at least one of the powders may be
irregular.
[0016] Particle roundness influences paste viscosity and the
tendency to shear. Spheres offer less resistance to viscous flow
compared with particles of irregular shape. Accordingly, paste made
from the same flux and spherical powder will have lower viscosity
than those of the same weight percent and particle size range but
of irregular shape. One possible advantage with pastes of the
latter appearance is that they are less likely to shear thin when
screen/stencil printed at high speed and constant squeegee motion.
Interlocking of the powder reduces paste flow out. Reduction of
shear thinning is important because it will prevent slumping and
smearing which can result in solder bridging and solder
balling.
[0017] Preferably the solder powder particles are from 1 to 100
microns in mean diameter. More preferably the particles are from 1
to 75 microns in mean diameter, most preferably 1 to 50 microns.
The diameter measurement refers to the longest diameter of the
particle. Preferably the powder particles of both the first and
second components are substantially the same.
[0018] Preferably the solder composition consists of the blend of
the first powder component and the second powder component,
together with unavoidable impurities. It will be appreciated that
the composition according to the present invention may contain
unavoidable impurities, possibly as part of the first and/or second
components, although, in total, these are unlikely to exceed 1 wt %
of the composition. Preferably, the alloys contain unavoidable
impurities in an amount of not more than 0.5 wt % of the
composition, more preferably not more than 0.3 wt % of the
composition, still more preferably not more than 0.1 wt % of the
composition.
[0019] Preferably the solder composition is lead-free. This allows
the composition to comply with regulatory requirements.
[0020] The inventors have discovered that it is possible to
engineer the effective melting temperature, and the mechanical,
electrical and thermal properties of a reflowed solder using
standard solder alloy and/or metal powders.
[0021] In particular, the inventors have discovered that a mixture
of two or more solder alloys is particularly useful. In particular,
where the first and second solder alloys have different melting
points, during the first reflow, which goes up to a peak
temperature of above the liquidus of the lower melting alloy but
below the solidus of the other powder, the high temperature alloy
powder particles dissolve quickly into the liquid phase of the low
temperature alloy.
[0022] As the mixing progresses, the composition of the solder is
quickly changing. This makes the solidification process highly
nonlinear because the liquidus temperature of the mixed composition
is also continuously increasing until the alloys are completely
mixed.
[0023] Preferably the melting points differ by at least 5.degree.
C. More preferably the melting points differ by at least
10.degree.C. The greater the difference in melting points, the more
pronounced the improved characteristics are that can be obtained
from the known solder compositions.
[0024] Preferably the first and second solder alloys comprise at
least one common element. This facilitates a quick dissolution of
one alloy into the other at temperatures close to or sometimes even
below the melting point of one of the alloys. Preferably the at
least one common element is tin.
[0025] For example a 20% of SAC305 powder mixed with 80% of
eutectic 42Sn58Bi results in increasing the liquidus of the final
composition to approximately 165.degree.C. from 138.degree. C. of
the original 42Sn58Bi. This is because the addition of Sn shifts
the alloy composition away from the SnBi eutectic. In addition,
small amounts of Ag and Cu coming in from SAC305 change the
microstructure of the alloys providing additional improvements in
the final solder properties.
[0026] These forgoing process changes occur during the first
reflow. Thus, the first reflow can be carried out at a lower
temperature than would be required for the final blend. Before
reflow it is a mixture of two separate alloys.
[0027] As a consequence of the forgoing composition it has been
found that the presence of the higher melting component leads to an
increase in liquidus temperature and, thus, a decrease in the
homologous temperature at same operating temperatures. This means
an automatic increase in thermal fatigue life and a decrease in
high temperature creep. The homologous temperature allows
comparison of different solder compositions.
[0028] For example, a solder having a working temperature range of
-55.degree. C. to 125.degree. C. and a melting (liquidus)
temperature of 183.degree. C. (456 K) is working at from
0.53T.sub.mp to 0.92T.sub.mp. Increasing the melting temperature to
195.degree. C. reduces this range to from 0.49 T.sub.mp to 0.85
T.sub.mp. Thus, the tensile strength, shear strength and modulus of
elasticity are improved.
[0029] In addition, the decrease in fraction of Bi content in the
solder improves its ductility. The presence of small amount of Ag
and Cu improves ductility, thermal and electrical conductivity and
refines the solder microstructure, leading to enhanced mechanical
properties.
[0030] In a preferred embodiment, the first powder component forms
about 80% by weight of the solder composition and is 42Sn 58Bi, and
wherein the second powder composition forms about 20% by weight of
the solder composition and is SAC305 (96.5% Sn, 0.5% Cu, 3% Ag).
Preferably the solder composition consists of the forgoing
components. It will be appreciated that while this example
represents a preferred composition, the final composition may be
selected from any alloys and mixtures in suitable proportions.
[0031] In an alternative aspect, the present inventors have
discovered that a mixed powder of a metal and a solder alloy powder
has surprising benefits. Without wishing to be bound by theory, it
is believed that during reflow, solder forms intermetallic bonds
with the metal particles. Under a single long reflow or multiple
reflow cycles, some of the metal from the metal particles is
dissolved into the bulk solder while the rest remains in its
original form. This results in a mixture of solder and metals
particles forming a composite structure. Thus, the metal powder
addition can improve compliance of the resultant solder joint, and
improve thermal and electrical conductivity. An example is copper
powder mixed with SnBi alloy. SnBi is a brittle alloy with
relatively poor thermal and electrical conductivity. Addition of Cu
particles in solder bulk improves it electrical and thermal
conductivity. Another example is addition of nano and micro sized
Ag particles to improve its mechanical strength and enhance
electrical and thermal conductivity.
[0032] However, during the initial reflow step, the composition
melts at the melting temperature of the solder alloy. As a
consequence, unique properties of the solder can be achieved while
still having an easy to melt and handle solder composition.
[0033] When the second powder component is a metal, it is
preferably an element selected from Cu, Ni, Al or Ag. Other metals
that may be present include one or more of Au, Cr, In, Sb, Sc, Y,
Zn, Ce, Co, Cu, Ge, Mn, and Ti or rare earth elements. Metal powder
size and level can be selected to tailor thermal, mechanical and
electrical properties of the final solder joint.
[0034] In the foregoing compositions, the second powder component
can have a particle size ranging from smaller than to larger than
the first solder powder. In one preferred embodiment, the particle
size of the second powder is substantially the same size as the
first solder powder. That is, the second powder comprises particles
are from 0.02 to 100 microns in mean diameter. More preferably the
particles are from 0.02 to 75 microns. Still more preferably the
particles are from 0.02 to 50 microns. In certain situations,
particle size between 0.02 and 5 microns are preferred. In one
embodiment, the particles, in particular the metal particles, are
preferably from 1 nm to 100 microns, more preferably from 10 nm to
100 microns. The metal particles may be from 10 microns to 100
microns. Alternatively, the metal particles may have a mean
diameter of from 100 microns to 1000 microns.
[0035] In a third aspect, the present inventors have discovered
that a mixed powder of two immiscible alloys with similar melting
temperature but a different Solid-Liquid phase transition is
advantageous. Thus the first and second solder alloys have similar
melting temperatures and are immiscible. For example, some Bi
containing alloys expand during liquid-solid transition (negative
Coefficient of thermal expansion (CTE)) while many others shrink
(positive CTE). The inventors have discovered that by mixing
particles of a positive CTE alloy that has similar melting
temperature with a negative CTE SnBi they can obtain a low-stress
solder joint formation. They have further discovered that this will
happen when the two alloys do not mix on heating (i.e. are
immiscible). If they dissolve in each other then the resultant
alloy could have its own characteristic transition.
[0036] In the foregoing compositions, the second powder component
preferably has particle sizes that are comparable with those of the
first powder component. That is, the second powder comprises
particles are from 1 to 100 microns in mean diameter. More
preferably the particles are from 1 to 75 microns in mean diameter,
most preferably 1 to 50 microns. Preferably the particle sizes of
the first and second powders are substantially the same since this
facilitates easy handling and mixing.
[0037] By similar melting temperatures it is preferably meant that
the first and second solder alloys have a melting temperature
within at most 25.degree. C. More preferably the first and second
solder alloys have a melting temperature within at most 10.degree.
C. and most preferably within 1.degree. C.
[0038] Preferably the coefficient of thermal expansion of the first
solder alloy is positive and the coefficient of thermal expansion
of the second solder alloy is negative.
[0039] In order to ensure that the powders are immiscible with each
other, preferably at least the second powder component has a
non-reactive coating layer. This allows for the use of known
powders to achieve the advantageous benefits of the present
invention.
[0040] Preferably the solder composition further comprises a
further powder component selected from materials such as carbides,
nitrides, oxides and carbon nanotubes, preferably selected from
Al.sub.2O.sub.3, SiO.sub.2, TiO, NiO and carbon nanotubes. These
components are preferably sized in accordance with the solder and
metal particles described herein. That is, preferably having a
longest average diameter on the micron scale; preferably from 0.02
to 100 microns.
[0041] These components have surprisingly been found to allow for
alloy microstructure modification after reflow. As a consequence,
the mechanical properties and thermal fatigue life of the alloy can
be improved.
[0042] According to a further aspect of the present invention,
there is provided a solder composition comprising a blend of a
first powder component and a second powder component, wherein the
first powder component is a first solder alloy and the second
powder component is selected from materials such as: carbides,
nitrides, oxides and carbon nanotubes, preferably selected from
materials such as: Al.sub.2O.sub.3, SiO.sub.2, TiO, NiO and carbon
nanotubes. Preferably, the second powder component is one or more
of Al.sub.2O.sub.3, SiO.sub.2, TiO, NiO and carbon nanotubes. The
components of this aspect correspond to those in the foregoing
aspects. For example, the first powder component for use in this
aspect may be the same as any first powder component described
herein.
[0043] According to a further aspect of the present invention,
there is provided a solderable paste comprising the solder
composition as described herein. That is, the paste comprises the
powder blend of the present invention together with a flux.
Suitable fluxes are well known in the art.
[0044] The compositions of the present invention may then be
processed into the form of a bar, a stick, a solid or flux cored
wire, a foil or strip, a pre-form, a pre-applied or free standing
film or solder spheres for use in ball grid array joints, or a
pre-formed solder piece or a reflowed or solidified solder
joint.
[0045] According to a further aspect of the present invention,
there is provided a method of forming the solder composition as
described herein, the method comprising mixing a first powder
component with a second powder component.
[0046] According to a further aspect of the present invention,
there is provided the use of the composition as described herein or
the solderable paste as described herein in a soldering method.
[0047] According to a further aspect of the present invention,
there is provided the use of the composition as described herein or
the solderable paste as described herein to form a soldered
joint.
[0048] In a further aspect, the present invention provides a
soldered joint comprising an alloy of the first to fifth
aspects.
[0049] In a further aspect, the present invention provides the use
of an alloy of the first to fifth aspects in a soldering method.
Such soldering methods include, but are not restricted to, wave
soldering, Surface Mount Technology (SMT) soldering, die attach
soldering, thermal interface soldering, hand soldering, laser and
RF induction soldering, and rework soldering.
[0050] In a further aspect, the present invention provides a solder
composition comprising a blend of a first component and a second
component, wherein the first component is a first solder alloy and
the second component is a second solder alloy or a metal. The
preferable features of the above described aspects of the present
invention are also preferable with regard to this aspect of the
present invention. The first and/or second component may be in the
form of a powder, paste, strip, foil, sphere, disc or a preform.
Preferably the first component is in the form of a paste.
[0051] In a further aspect, the present invention provides a method
of forming the above described solder composition, the method
comprising mixing. Preferably the first component is a paste and/or
the second component is in the form of a powder, paste, strip,
foil, sphere, disc or a preform.
[0052] In a further aspect the present invention provides a method
of forming a solder joint comprising:
[0053] (i) providing two or more work pieces to be joined;
[0054] (ii) providing a first solder component having a first
reflow temperature;
[0055] (iii) providing a second solder component having a second
reflow temperature that is higher than said first reflow
temperature; and
[0056] (iv) heating said first and second solder components in the
vicinity of the work pieces to be joined, wherein said heating is
carried out at or above the first reflow temperature and below the
second reflow temperature.
[0057] The advantages in relation to the first to fifth aspects of
the present invention described above are also exhibited by the
method of this aspect of the present invention.
[0058] The work pieces to be joined may be, for example, a circuit
board and a circuit component. The method may be used, for example,
in the manufacture of printed circuit boards. The first solder
component may be a first alloy component, and may be in the form of
a powder, paste, strip, foil, sphere, disc or a preform, preferably
a paste. The second solder component may be a second solder alloy
or a metal, and may be in the form of a powder, paste, strip, foil,
sphere, disc or a preform. Once the solder components have been
mixed, they may be heated at a temperature lower than the reflow
temperature of the first solder component. An example of the above
described method is as follows:
[0059] A method of assembly, comprising:
[0060] applying solder paste to a printed circuit board to form a
solder paste deposit;
[0061] placing a low temperature preform in the solder paste
deposit;
[0062] processing the printed circuit board at a reflow temperature
of the solder paste to create a low temperature solder joint;
and
[0063] processing the low temperature solder joint at a reflow
temperature that is lower than the reflow temperature of the solder
paste.
[0064] The invention will now be described with reference to the
following non-limiting examples.
[0065] A solder composition was prepared comprising a 42Sn 58Bi
powder component in an amount of about 80% by weight of the solder
composition and about 20% by weight of a SAC305 powder (96.5% Sn,
0.5% Cu, 3% Ag). On testing it was found that the alloy had
improved ductility, thermal fatigue and creep resistance compared
to the 42Sn 58Bi powder alone.
[0066] A solder composition was prepared comprising a 42Sn 58Bi
powder component in an amount of about 80% by weight of the solder
composition and about 20% by weight of a Copper metal powder. On
testing it was found that the alloy had improved ductility, thermal
fatigue resistance and electrical conductivity compared to the 42Sn
58Bi powder alone.
[0067] A solder composition was prepared comprising two
bismuth-containing alloys. One of the alloys selected expands
during liquid-solid transition (-ve CTE) and the other shrinks (+ve
CTE). This composition was found to give rise to a low-stress
solder joint.
[0068] Two solder compositions were prepared. The first comprised
82.9 wt % SAC305 and 17.1 wt % Sn58Bi, and the second contained
82.9 wt % SACX0307 (Sn0.3Ag0.7Cu0.1Bi) and 17.1 wt % Sn58Bi.
Measurement of the chip shear resistance and the pin pull
resistance indicated that the values were comparable with the
benchmark alloy Sn57.6Bi0.4Ag.
[0069] The present application includes, by way of example, the
following figures:
[0070] FIGS. 1A and 1B show differential scanning calorimetry (DSC)
traces of the melting of two solder compositions (samples sizes:
29.1000 mg and 29.3000 mg, respectively; instrument: 2920 DSC
V2.6A). The first is a mixture of 20% SAC and 80% Sn58Bi. The
second is Sn45Bi. These traces are similar, although SAC has a
melting point of 217.degree. C. The SAC dissolves into the Sn58Bi
well below the melting temperature of SAC. At the same time, it has
been found that the first mixture shows significantly higher shear
force in a ball shear test than Sn58Bi alone (949 vs 911). DSC
traces were also obtained for a mixture of 17.1% Sn58Bi and 82.9%
Sn0.3Ag0.7Cu0.1 Bi (SACX0307). The initial scan showed a low
temperature peak corresponding to the melting of SnBi alloy.
However, this peak disappeared on subsequent scans, indicating that
all low temperature phase is converted into high temperature phase
by dissolving SACX0307 into liquid Sn58Bi.
[0071] FIGS. 2A and 2B show the improvement in the elastic modulus
of SnBi on the addition of nano or micro sized Ag particles. In the
former, the particles are nano sized, having an average particle
size of from 20 nanometers to 1 micron. In the latter, the AG
particles are from 1 micron to 100 microns in size. As can be seen,
even a small amount (1%) of AG particles has been found to have a
significant effect on the elastic modulus. Addition of silver
particles improves thermal and electrical conductivities of the
solder. Presence of free silver particles in the Sn58Bi solder has
surprisingly been found to increase its thermal conductivity by
more than 50%. Furthermore, the silver addition changes the alloy
microstructure. Even up to 5% Ag addition has surprisingly been
found to not result in long Ag.sub.3Sn crystals. In FIG. 2A elastic
modulus values are shown for the following solders (from left to
right): Sn58Bi, a mixture of Sn58Bi+1% nano sized Ag, a mixture of
Sn58Bi+3% nano sized Ag and a mixture of Sn58Bi+5% nano sized Ag.
In FIG. 2B elastic modulus values are shown for the following
solders (from left to right): Sn58Bi, a mixture of Sn58Bi+1% micron
sized (from 1 to 100 microns) Ag, a mixture of Sn58Bi+3% micron
sized Ag and a mixture of Sn58Bi+5% micron sized Ag.
[0072] FIG. 3 shows a comparison of the shear strengths of a
standard Sn58Bi solder (left hand side) and that of a solder
composition according to the present invention (Sn58Bi+20% SAC305).
Addition of SAC305 powder to Sn58Bi powder results in a final
composition with lower Bi after reflow and which also shows a
higher shear strength
[0073] FIGS. 4a-c show a range of micrographs showing the crystal
structure of a number of solder compositions as described herein.
FIGS. 4a and 4b show the microstructure of Sn45Bi and Sn58Bi alloys
respectively, each with the addition of Al.sub.2O.sub.3. In each
case, Alumina powder was added to a paste flux which was printed on
a copper coupon. Thin preforms of Sn45Bi and Sn58Bi were placed on
the flux. Heated on a hot plate at 185 C and cooled in air. Due to
diffusion of Alumina particles into the solder, the solder
microstructure is markedly refined near the interface.
[0074] FIG. 4c shows a micrograph of an Sn58Bi alloy with copper
particles. As can be seen, copper particles are uniformly
distributed in the SnBi alloy matrix. A CuSn IMC layer is seen at
the surface of the particles but bulk of the particles is pure
copper.
[0075] FIGS. 5a and 5b demonstrate the addition of Nickel into a
Sn45Bi solder alloy. In FIG. 5a, there is no Ni present. In FIG.
5b, there is included 0.02 Ni and this has a significant grain
refining effect.
[0076] FIGS. 6a, 6b and 6c indicate the changes in shear force
(6a), pull force (6b) and intermetallic compound (IMC) growth (6c)
during thermal cycling for a mixture of Sn58Bi+22.4 wt % SAC305
(diamonds), a mixture of Sn58Bi+22.4 wt % SACX0307 (squares) and
Sn45Bi (triangles). The thermal cycling conditions were from -40 to
125.degree. C., 10 minutes dwell time and 1000 cycles. FIGS. 6a and
6b indicate that the shear force and pull force (lead pull
resistance) values of the solders of the present invention decrease
less after thermal cycling that Sn45Bi. FIG. 6c indicates that IMC
growth during thermal cycling is much lower for the solders of the
present invention compared to Sn45Bi, which indicates a much better
solder joint reliability for the solders of the present
invention.
[0077] FIG. 7 shows drop shock resistance data for a mixture of
Sn58Bi+SAC305 (circles) and Sn45Bi (squares). The drop shock
resistance of the mixture of Sn58Bi+SAC305 is clearly higher
(average number of drops to failure: 200.3) than that of Sn45Bi
(average number of drops to failure: 167.2).
[0078] FIG. 8a demonstrates the shear strength values for the
alloys (from left to right): Sn58Bi (as cast), Sn58Bi (48 hours
after casting), a mixture of Sn58Bi+1 wt % micron sized Ag
particles (48 hours after casting), a mixture of Sn58Bi+3 wt %
micron sized Ag particles (48 hours after casting), a mixture of
Sn58Bi+1 wt % micron sized Ag coated Cu particles (48 hours after
casting), a mixture of Sn58Bi+3 wt % micron sized Ag coated Cu
particles (48 hours after casting) and a mixture of Sn58Bi+5 wt %
micron sized Ag coated Cu particles (48 hours after casting). The
addition of Ag recovers the shear strength that is lost as a result
of aging (an increase of 14.6% for 3 wt % Ag particles).
[0079] FIG. 8b demonstrates the hardness values for the alloys
(from left to right): Sn58Bi, a mixture of Sn58Bi +1 wt % micron
sized Ag particles, a mixture of Sn58Bi+3 wt % micron sized Ag
particles, a mixture of Sn58Bi+5 wt % micron sized Ag particles, a
mixture of Sn58Bi +1 wt % nanometer sized Ag particles, a mixture
of Sn58Bi+3 wt % nanometer sized Ag particles, and a mixture of
Sn58Bi+5 wt % nanometer sized Ag particles, The hardness increases
up to 25% with the addition of 3 wt % micron sized Ag
particles.
[0080] The presently claimed compositions are useful for
applications including, but not limited to, LED assembly,
photovoltaic cell tabbing and stringing, semiconductor backend
process, and Die attachment. Final form factor is application
dependent but the solder can be made into any form, including, but
not limited to, paste, perform, film, and wire, and can be combined
with cleanable or no-clean flux chemistry.
[0081] When introducing elements of the present disclosure or the
preferred embodiments(s) thereof, the articles "a", "an", "the" and
"said" are intended to mean that there are one or more of the
elements. The terms "comprising", "including" and "having" are
intended to be inclusive and mean that there may be additional
elements other than the listed elements.
[0082] The foregoing detailed description has been provided by way
of explanation and illustration, and is not intended to limit the
scope of the appended claims. Many variations in the presently
preferred embodiments illustrated herein will be apparent to one of
ordinary skill in the art, and remain within the scope of the
appended claims and their equivalents.
* * * * *